The present invention relates generally to battery-powered circuits for LEDs, and particularly to a system and method of driving LEDs.
Rechargeable batteries are utilized as a power source in a wide variety of electronic devices. In particular, rechargeable batteries are utilized in portable consumer electronic devices such as cellular telephones, portable computers, Global Positioning System (GPS) receivers, and the like. Many of these devices employ a rechargeable lithium ion battery, with a typical output voltage in the range of 3V to 4.2V.
A fairly recent development in solid state electronics is the development of the white-light LED. White LEDs offer significant advantages over alternative white-light sources, such as small incandescent bulbs or fluorescent lights. Among these are greater efficiency (resulting in lower heat generation and lower power consumption for a given level of illumination), increased operating life, and superior ruggedness and shock resistance. White LEDs are often employed in portable electronic devices, such as to back-light an LCD display screen. Like all LEDs, the intensity of light emitted by a white LED varies as a function of the DC current through it. In many applications, it is highly desirable to allow the user to adjust or select the light intensity. Additionally, where a plurality of white LEDs are employed, it is often desirable that they all be driven to the same intensity level.
The forward voltage drop of a white light LED is typically in the range of 3V to 3.8V. As this voltage drop is close to, or may exceed, the output voltage of a lithium ion battery, power for white LEDs is typically supplied from the battery through a DC-DC boost converter, such as a charge pump. These converters boost the output voltage of the battery to a level much greater than the forward voltage of the white LEDs. While this provides sufficient drive to power the LEDs, the inefficiency of the boost converter potentially wastes limited battery power.
With increasing power management sophistication, circuit miniaturization, low ambient power circuits, and the reduced bandwidth of many digital communications, portable electronic devices are often operated in a variety of “low-power” modes, wherein some features and/or circuits are inactive or operate at a reduced capacity. As one example, many newer cellular telephones include an “internet mode,” displaying text data (such as on an LCD screen) that is transmitted at a very low data rate as compared to voice communications, thus consuming low levels of power and extending battery life. A typical current budget for a cellular telephone in this mode is around 200 mA. Such a phone typically utilizes three white LEDs, at 20 mA each, to back-light the display. The LED current thus accounts for approximately 30% of the total battery current. In such an application, an efficient method of supplying power to the LEDs would have a significant effect on battery life.
Another challenging issue facing designers is that the forward voltage drop of white LEDs varies significantly. For example, two LEDs chosen at random from the same production run could have forward voltages that vary by as much as 200 mV. Thus, an efficient current supply design for biasing white LEDs, which preserves good current matching between diodes with different forward voltages, would represent a significant advance in the state of the art, as it would ensure uniform illumination.
FIG. 1 depicts a typical discharge pattern of a lithium ion battery. Curve 1 represents the battery discharge pattern at an ambient temperature of 25° C.; curve 2 represents the battery discharge profile at an ambient temperature of 35° C. As FIG. 1 illustrates, while the output of a lithium ion battery may vary between approximately 2.5V and 4.2V, for approximately 95% of the lithium ion battery's lifetime, its output voltage exceeds 3.5V. Thus, if the battery is driving white LEDs with forward voltages of less than approximately 3.5V, it should be possible to drive the diodes directly from the battery, obviating the need to boost the battery output by a DC-DC converter.
In practice, this is problematic for at least two reasons. First, each white LED current source must impose only a very small voltage drop, and regulate a current value that may vary over an order of magnitude or more for brightness control. In addition, each LED will require a separate current source, due to the wide variation in forward voltage drops across white LEDs.
Second, as the battery output voltage drops towards the end of the battery's lifetime, a provision must be made for first detecting this condition, and then boosting the battery output to provide sufficient current to power all white LEDs at the required intensity level.